U.S. officials believe the suicide bomber who killed Israeli tourists in Burgas, Bulgaria, was a member of Hezbollah.

The bomber was “acting under broad guidance” to hit Israeli targets, an unnamed senior American official told The New York Times. Other American officials confirmed that the bomber was a member of Hezbollah, according to the newspaper.

The bomber, who killed five Israelis, a Bulgarian bus driver and himself, reportedly had a fake Michigan driver’s license. While his name or nationality remains unknown, there are no indications that he was connected to the United States, the senior American official said.

“This looks like he was hanging out for a local target, and when this popped up he jumped on it,” the official said, according to the newspaper.

He speculated that the attack was in response for assassinations of Iranian nuclear scientists and workers in the past few years, which Iran has blamed on Israel. Israel has neither confirmed nor denied the allegations.

Shortly after the bombing, Israeli Prime Minister Benjamin Netanyahu and Defense Minister Ehud Barak blamed Hezbollah for the attack.